After a lot of beta testing I came to the conclusion that if I erase the creation of the cookie from index.php the problem is not solved. But if I erase it after the creation everything is ok.

It occurs in both Yes or No options at General Settings -> Set cookie to prevent repeated participation, which I think is the start of the problem.

Now MAYBE THE SOLUTION:
When I click "Exit and clear survey" and start again everything works. So trying to go directly to the url h++p://survey/index.php?sid=28836&newtest=Y seems to solve the problem.